Participants are classified according to their exposure to a factor of interest and then are observed over time to document the incidence of disease in what type of study?

This research study would be a prospective cohort study. 

In this study, the groups are different in some way, for example, a control group which received no treatment, a standard group who received a well-studied drug and a test group who received a new drug. These groups are followed over time and the outcome (such as curing a disease or alleviating symptoms) will be compared. 

the correct answer is:

a population changes in response to a change in the environment.